auth: remove `auth revoke`; redefine `logout --all` to revoke sessions

Delete the entire auth revoke command. Session management collapses to two verbs: auth status shows active sessions, logout ends them.

Redefine the logout --all flag — it no longer removes all local contexts. Instead:

• logout revokes the active session server-side (DELETE .../tokens/current) and removes the active context locally. (Unchanged default behaviour.)
• logout --all additionally asks the server to revoke every session on the active core (list families -> delete each by id). The local side is identical to the default.

After a logout clears the active context, the next saved context is promoted to active, so running entire logout repeatedly drains every saved login in turn.

Cross-core revoke is out of scope: these endpoints target AuthBaseURL's core only, pending the COR-389 control-plane retargeting.